How do I delete a Grease Pencil ruler?
Pressing Delete will delete the active ruler (whichever is selected or left-click to select a specific one) before having pressed enter (all the options in the help line are options for during the creation of the ruler). 

If you have already pressed Enter you can re-click the grease pencil 'Ruler/Protractor' button to re-enable the rulers and go through the selection and delete process described above.

After pressing Enter the rulers are saved to a grease pencil layer, so you could delete all rulers at once via the grease pencil panel in the properties menu (N) in the 3D view by selecting the grease pencil layer in the list and then pressing the ' - ' (minus) button: